PCB/FPGA Power Architectures, Part 1

The first stage in defining the power architecture of a system is a determination of all the voltage rails and the currents drawn by each of these rails.

Based on the wealth of comments garnered by my recent article on becoming a decoupling capacitor guru, I thought I would write a little more on the hardware/system aspects of designing a FPGA and using a FPGA in your system. Of course, a lot of this is applicable to non-FPGA-based systems.

One of the more interesting aspects to start looking at is the power architecture of the design. How do we go about powering the FPGA (and other devices) on the board? Normally, the system will have an intermediate voltage, which comes from an AC/DC convertor or another form of DC supply that powers the system. The first stage of the design is to specify this interface correctly in terms of the design's voltage and current requirements. Determining this intermediate voltage is the easier task of the two, as the required current will have to account for any inefficiencies in the downstream convertor(s).

The first stage in defining the power architecture is a determination of all the voltage rails and the currents drawn by each of these rails. For example, consider the FPGA-based imaging system as shown below:

In the case of such a system, you may have a number of voltage rails, such as the ones shown in the following table:

For this particular example, let's assume that all of the power supplies have a requirement to be within ±5%. As can be seen from the above table, the highest voltage is 3.465 V, which is the nominal 3.3 V at its maximum acceptable tolerance. Knowing this value allows us to determine the voltage supplied by the AC/DC or other DC supply within the system. The sensible thing to do here is to select a convertor that has an output compatible with the 3.3 V required, thereby saving a conversion stage and increasing the overall efficiency.

The next stage is to determine the power required by each of the rails. This requires that you use power estimation tools such as Xilinx XPE and read the datasheets for other devices to ensure you can determine the total amount of power required. I tend to collate all of this in a spreadsheet, as this comes in useful later on when we are determining the conversion architectures.

As you can see above, when I calculated the power required by the board, I performed two calculations -- one for the nominal and one for the maximum power. This is because, at this point in time, I have not yet calculated the maximum rail voltages provided in the worst case by the convertors. Therefore, I have assumed they will be at maximum voltage. This is important, since it is needed to determine the power required in the worst case by the AC/DC convertor (you should always design to address worst case requirements). While the difference (146.5 mW in the example above) is not large in this case, it could be in a larger system.

Having determined the load power, we need to determine the overall power requirements -- including loses in the power convertors -- before we can specify the power required from the AC/DC convertor or DC supply.

Having determined the power required by each device, the next step is to determine the power required by each rail. This can subsequently be used to determine the conversion architecture, although -- of course -- other requirements also come into play when determining this.

When it comes to the power architecture itself, there are two main types of convertors as follows:

Switching regulators generate the regulated output voltage by switching storage inductors into, and out of, the circuit to maintain a regulated output voltage. This switching is controlled via either an analog or digital control loop. With a switching regulator, 100% efficiency is theoretically achievable. In the real world, however, components are not ideal, but efficiencies greater than 90% can be achieved and Gallium Nitride (GaN) power FETs promise even better performance.

Linear regulators generate the regulated voltage by dissipating the excess power across a pass transistor. This dissipation is managed via a control loop to adjust for fluctuations. Since there is no switching involved, the linear regulator is often used where quieter power supplies are required. However, this does not mean that all ripple on the voltage rail is rejected. As can be seen in the image below, the ripple rejection decreases as the frequency increases:

In my next column on this topic, I will consider the advantages and disadvantages of switching regulators versus their linear counterparts, along with other considerations that must be taken into account to obtain a good, efficient, and reliable power architecture. In the meantime, I welcome your questions and comments.